Jean-François Halet

Ruthenium complexes of C,C '-bis(ethynyl)carboranes: an investigation of electronic interactions mediated by spherical pseudo-aromatic spacers

The complexes [Ru(1-C[triple bond]C-1,10-C2B8H9)(dppe)Cp*] (3a), [Ru(1-C[triple bond]C-1,12-C2B10H11)(dppe)Cp*] (3b), [{Ru(dppe)Cp*}2{mu-1,10-(C[triple bond]C)2-1,10-C2B8H8}] (4a) and [{Ru(dppe)Cp*}2{mu-1,12-(C[triple bond]C)2-1,12-C2B10H10}] (4b), which form a representative series of mono- and bimetallic acetylide complexes featuring 10- and 12-vertex carboranes embedded within the diethynyl bridging ligand, have been prepared and structurally characterized. In addition, these compounds have been examined spectroscopically (UV-vis-NIR, IR) in all accessible redox states. The significant separation of the two, one-electron anodic waves observed in the cyclic voltammograms of the bimetallic complexes 4a and 4b is largely independent of the nature of the electrolyte and is attributed to stabilization of the intermediate redox products [4a]+ and [4b]+ through interactions between the metal centers across a distance of ca. 12.5 A. The mono-oxidized bimetallic complexes [4a]+ and [4b]+ exhibit spectroscopic properties consistent with a description of these species in terms of valence-localized (class II) mixed-valence compounds, including a unique low-energy electronic absorption band, attributed to an IVCT-type transition that tails into the IR region. DFT calculations with model systems [4a-H]+ and [4b-H]+ featuring simplified ligand sets reproduce the observed spectroscopic data and localized electronic structures for the mixed-valence cations [4a]+ and [4b]+.

Electron‐Rich Iron/Ruthenium Arylalkynyl Complexes for Third‐Order Nonlinear Optics: Redox‐Switching between Three States

The new [(η(2)-dppe)(η(5)-C(5)Me(5))Fe(C≡C-1,4-C(6)H(4)C≡C)Ru(η(2) -dppe)(2) C≡C(C(6)H(5))] complex (3-H) and its hexanuclear relative [{(η(2)-dppe)(η(5)-C(5) Me(5))Fe(C≡C-1,4-C(6)H(4)-C≡C)Ru(η(2)-dppe)(2)(C≡C-1,4-C(6)H(4)C≡C)(3)(1,3,5-C(6)H(3))] (4) have been synthesized and characterized. The linear and cubic nonlinear optical properties of these compounds in their various redox states have been studied along with those of the analogous complexes [(η(2)-dppe)(η(5)-C(5)Me(5))Fe(C≡C-1,4-C(6)H(4)C≡C)Ru(η(2)-dppe)(2)R][PF(6)](n) (n=0-2; R=Cl, 2-Cl; R=C≡C(4-C(6)H(4)NO(2)),3-NO(2)). We show that molecules exhibiting large third-order nonlinearities can be obtained by assembling such dinuclear Fe/Ru units around a central 1,3,5-substituted C(6)H(3) core. These data are discussed with a particular emphasis on the large changes in their nonlinear (third-order) optical properties brought about by oxidation. Experimental and computational (DFT) evidence for the electronic structures of these compounds in their various redox states is presented using 3-H(n+) as a prototypical model. Single crystals of this complex in its mono-oxidized state (3-H[PF(6)]) provide the first structural data for such carbon-rich Fe(III) /Ru(II) heteronuclear mixed-valent (MV) systems. Although experimental evidence for the structure of the dioxidized states was more difficult to obtain, the theoretical study reveals that 3-H(2+) can be considered to have a biradical structure with two independent spins. The low-lying absorptions that appear in the near-infrared (NIR) range for all these compounds following oxidation correspond to intervalence charge-transfer (IVCT) bands for the mono-oxidized states and to ligand-to-metal charge-transfer (LMCT) transitions for the dioxidized states. These play a crucial role in the strong optical modulation achieved. The possibility of accessing additional states with distinct linear or nonlinear optical properties is also briefly discussed.

3,5-Bis(ethynyl)pyridine and 2,6-bis(ethynyl)pyridine spanning two Fe(Cp*)(dppe) units: role of the nitrogen atom on the electronic and magnetic couplings.

The role of the nitrogen atom on the electronic and magnetic couplings of the mono-oxidized and bi-oxidized pyridine-containing complex models [2,6-{Cp(dpe)Fe-C≡C-}(2)(NC(5)H(3))](n+) and [3,5-{Cp(dpe)Fe-C≡C-}(2)(NC(5)H(3))](n+) is theoretically tackled with the aid of density-functional theory (DFT) and multireference configuration interaction (MR-CI) calculations. Results are analyzed and compared to those obtained for the reference complex [1,3-{Cp*(dppe)Fe-C≡C-)}(2)(C(6)H(4))](n+). The mono-oxidized species show an interesting behavior at the borderline between spin localization and delocalization and one through-bond communication path among the two involving the central ring, is favored. Investigation of the spin state of the dicationic complexes indicates ferromagnetic coupling, which can differ in magnitude from one complex to the other. Very importantly, electronic and magnetic properties of these species strongly depend not only upon the location of the nitrogen atom in the ring versus that of the organometallic end-groups but also upon the architectural arrangement of one terminus, with respect to the other and/or vis-à-vis the central ring. To help validate the theoretical results, the related families of compounds [1,3-{Cp*(dppe)Fe-C≡C-)}(2)(C(6)H(4))](n+), [2,6-{Cp*(dppe)Fe-C≡C-}(2)(NC(5)H(3))](n+), [3,5-{Cp*(dppe)Fe-C≡C-}(2)(NC(5)H(3))](n+) (n = 0-2) were experimentally synthesized and characterized. Electrochemical, spectroscopic (infrared (IR), Mössbauer), electronic (near-infrared (NIR)), and magnetic properties (electron paramagnetic resonance (EPR), superconducting quantum interference device (SQUID)) are discussed and interpreted in the light of the theoretical data. The set of data obtained allows for many strong conclusions to be drawn. A N atom in the long branch increases the ferromagnetic interaction between the two Fe(III) spin carriers (J > 500 cm(-1)), whereas, when placed in the short branch, it dramatically reduces the magnetic exchange in the di-oxidized species (J = 2.14(5) cm(-1)). In the mixed-valence compounds, when the N atom is positioned on the long branch, the intermediate excited state is higher in energy than the different ground-state conformers and the relaxation process provides exclusively the Fe(II)/Fe(III) localized system (H(ab) ≠ 0). Positioning the N atom on the short branch modifies the energy profile and the diabatic mediating state lies just above the reactant and product diabatic states. Consequently, the LMCT transition becomes less energetic than the MMCT transition. Here, the direct coupling does not occur (H(ab) = 0) and only the coupling through the bridge (c) and the reactant (a) and product (b) diabatic states is operating (H(ac) = H(bc) ≠ 0).

Syntheses, structures, two-photon absorption cross-sections and computed second hyperpolarisabilities of quadrupolar A–π–A systems containing E-dimesitylborylethenyl acceptors

A series of bis(E-dimesitylborylethenyl)-substituted arenes, namely arene = 1,4-benzene, 1,4-tetrafluorobenzene, 2,5-thiophene, 1,4-naphthalene, 9,10-anthracene, 4,4′-biphenyl, 2,7-fluorene, 4,4′-E-stilbene, 4,4′-tolan, 5,5′-(2,2′-bithiophene), 1,4-bis(4-phenylethynyl)benzene, 1,4-bis(4-phenylethynyl)tetrafluorobenzene and 5,5″-(2,2′:5′,2″-terthiophene), have been synthesised viahydroboration of the corresponding diethynylarenes with dimesitylborane. Their absorption and emission maxima, fluorescence lifetimes and quantum yields are reported along with the two-photon absorption (TPA) spectra and TPA cross-sections for the 5,5′-bis(E-dimesitylborylethenyl)-2,2′-bithiophene and 5,5′-bis(E-dimesitylborylethenyl)-2,2′:5′,2″-terthiophene derivatives. The TPA cross-section of the latter compound of ca. 1800 GM is the largest yet reported for a 3-coordinate boron compound and is in the range of the largest values measured for quadrupolar compounds with similar conjugation lengths. The X-ray crystal structures of 1,4-benzene, 2,5-thiophene, 4,4′-biphenyl and 5,5″-(2,2′:5′,2″-terthiophene) derivatives indicate π-conjugation along the BCC–arene–CCB chain. Theoretical studies show that the second molecular hyperpolarisabilities, γ, in each series of compounds are generally related to the HOMO energy, which itself increases with increasing donor strength of the spacer. A strong enhancement of γ is predicted as the number of thiophene rings in the spacer increases.

Chemistry of the 1,3,5,7-octatetraynediyl carbon rod end-capped by two electron-rich (η5-C5Me5)(η2-dppe)Fe groups

Abstract The synthesis of the organoiron complex [(η5-C5Me5)(η2-dppe)FeCCCCCCCCFe(η2-dppe)(η5-C5Me5)] (2, dppe=1,2-bis(diphenylphosphino)ethane) is reported with its full spectroscopic characterizations (1H-, 31P-, and 13C-NMR, IR, Raman, UV–vis and 57Fe Mossbauer). The X-ray analysis of 2 shows that the molecule adopts a geometry very close to the anti conformation in the solid state. The shortening of the FeC bond distance associated with the increase in the number of carbon atoms suggests some cumulenic contribution to the description of the electronic structure of the all-carbon bridge. The 13C-NMR, 57Fe Mossbauer data and theoretical calculations confirm this trend and indicate that the cumulenic contribution is significant in the vicinity of the metal center but vanishes in the middle of the carbon rod. Vibrational spectroscopy carried out on the single crystals of [(η5-C5Me5)(η2-dppe)FeCCCCFe(η2-dppe)(η5-C5Me5)] (1) and 2 and on solutions of these compounds indicates that the CC bond stretching mode is not very sensitive to the relative orientation of the terminal endgroups. The electronic structure of the titled compound has been investigated using density functional theory. The geometrical changes occurring upon elongation of the carbon chain were nicely reproduced and interpreted. Time-dependant density functional theory calculations have been performed to rationalize the optical spectra.

DFT calculations of quadrupolar solid‐state NMR properties: Some examples in solid‐state inorganic chemistry

This article presents results of first‐principles calculations of quadrupolar parameters measured by solid‐state nuclear magnetic measurement (NMR) spectroscopy. Different computational methods based on density functional theory were used to calculate the quadrupolar parameters. Through a series of illustrations from different areas of solid state inorganic chemistry, it is shown how quadrupolar solid‐state NMR properties can be tackled by a theoretical approach and can yield structural information.

Electronic structure of the new rare earth borocarbide Sc2BC2

Abstract The electronic structure and bonding in the new ternary borocarbide Sc2BC2 are analysed using extended Huckel calculations. A rather strong covalency is observed between the carbon atoms of the isolated BC2 entities and the scandium atoms. The metallic behaviour of Sc2BC2 occurs mainly through scandium states which form the bottom of the conduction band. Resistivity is expected to be greater along the c axis. The effect of the addition of electrons to the system is also analysed.

Reactions of M(CCCCR)(CO)3Cp [M=Mo, W; R=H, Fe(CO)2Cp, M(CO)3Cp] with cobalt carbonyls: X-ray structures of {Cp(OC)8Co2M(μ3-C)}CC{(μ3-C)Co2M′(CO)8Cp} (M=M′=Mo, W; M=Mo, M′=W)

Abstract Reactions of Co2(CO)8 with complexes M(CCCCR)(CO)3Cp [M=Mo, W; R=H, Fe(CO)2Cp] are described. Simple adducts containing a Co2(CO)6 group attached to the least sterically-hindered CC triple bond are formed. In contrast, when R=M(CO)3Cp (M=Mo, W), bis-cluster complexes {Cp(OC)8Co2M(μ3-C)}CC{(μ3-C)Co2M′(CO)8Cp} (M=M′=Mo, W; M=Mo, M′=W) were obtained. All three complexes were structurally characterised. Important features are the presence of both distal and proximal Cp groups in each molecule, and the formal oxidation of the –CC–CC– chain in the precursor to C–CC–C system in the products. Extended Huckel and Density Functional Theory calculations have been used to rationalise the observed structures of the M3C4M3 complexes. The reaction between Co2(μ-dppm)(CO)6 and {W(CO)3Cp}2(μ-C4) gave the simple adduct Co2(μ-dppm){μ-[Cp(OC)3W]C2CC[W(CO)3Cp]}(CO)4.

Experimental and Theoretical Studies of Quadrupolar Oligothiophene-Cored Chromophores Containing Dimesitylboryl Moieties as π-Accepting End-Groups: Syntheses, Structures, Fluorescence, and One- and Two-Photon Absorption

Quadrupolar oligothiophene chromophores composed of four to five thiophene rings with two terminal (E)-dimesitylborylvinyl groups (4 V-5 V), and five thiophene rings with two terminal aryldimesitylboryl groups (5 B), as well as an analogue of 5 V with a central EDOT ring (5 VE), have been synthesized via Pd-catalyzed cross-coupling reactions in high yields (66-89%). Crystal structures of 4 V, 5 B, bithiophene 2 V, and five thiophene-derived intermediates are reported. Chromophores 4 V, 5 V, 5 B and 5 VE have photoluminescence quantum yields of 0.26-0.29, which are higher than those of the shorter analogues 1 V-3 V (0.01-0.20), and short fluorescence lifetimes (0.50-1.05 ns). Two-photon absorption (TPA) spectra have been measured for 2 V-5 V, 5 B and 5 VE in the range 750-920 nm. The measured TPA cross-sections for the series 2 V-5 V increase steadily with length up to a maximum of 1930 GM. We compare the TPA properties of 2 V-5 V with the related compounds 5 B and 5 VE, giving insight into the structure-property relationship for this class of chromophore. DFT and TD-DFT results, including calculated TPA spectra, complement the experimental findings and contribute to their interpretation. A comparison to other related thiophene and dimesitylboryl compounds indicates that our design strategy is promising for the synthesis of efficient dyes for two-photon-excited fluorescence applications.

Synthesis, photophysics and molecular structures of luminescent 2,5-bis(phenylethynyl)thiophenes (BPETs)

The Sonogashira cross-coupling of two equivalents of para-substituted ethynylbenzenes with 2,5-diiodothiophene provides a simple synthetic route for the preparation of 2,5-bis(para-R-phenylethynyl)thiophenes (R = H, Me, OMe, CF3, NMe2, NO2, CN and CO2Me) (1a–h). Likewise, 2,5-bis(pentafluorophenylethynyl)thiophene (2) was prepared by the coupling of 2,5-diiodothiophene with pentafluorophenylacetylene. All compounds were characterised by NMR, IR, Raman and mass spectroscopy, elemental analysis, and their absorption and emission spectra, quantum yields and lifetimes were also measured. The spectroscopic studies of 1a–h and 2 show that both electron donating and electron withdrawing para-subsituents on the phenyl rings shift the absorption and emission maxima to lower energies, but that acceptors are more efficient in this regard. The short singlet lifetimes and modest fluorescence quantum yields (ca. 0.2–0.3) observed are characteristic of rapid intersystem crossing. The single-crystal structures of 2,5-bis(phenylethynyl)thiophene, 2,5-bis(para-carbomethoxyphenylethynyl)thiophene, 2,5-bis(para-methylphenylethynyl)thiophene and 2,5-bis(pentafluorophenylethynyl)thiophene were determined by X-ray diffraction at 120 K. DFT calculations show that the all-planar form of the compounds is the lowest in energy, although rotation of the phenyl groups about the CC bond is facile and TD-DFT calculations suggest that, similar to 1,4-bis(phenylethynyl)benzene analogues, the absorption spectra in solution arise from a variety of rotational conformations. Frequency calculations confirm the assignments of the compounds’ IR and Raman spectra.